As the integration of semiconductor devices becomes high, a design rule for the semiconductor device gradually decreases so that an opening or a contact hole for electrically connecting metal wirings, such as a bit line, also has a high aspect ratio. When the metal wiring including a metal barrier layer and a metal plug is formed in the opening or the contact hole, a metal layer can have good step coverage and low contact resistance to improve reliability and performance of the semiconductor device.
The contact resistance of the semiconductor device generally decreases when a resistance reduction layer is provided between the metal plug and a semiconductor substrate. The resistance reduction layer may be called as an ohmic layer. The ohmic layer can include metal silicide such as cobalt silicide.
It is known to form a cobalt silicide layer using two heat treatment processes to form metal wiring that includes the cobalt silicide layer. Particularly, after a cobalt layer is formed on a substrate, the cobalt layer is thermally treated by a primary heat treatment process to convert the cobalt layer into a cobalt silicide layer. Then, the cobalt silicide layer is thermally treated by a secondary heat treatment process because the cobalt silicide layer has high resistance. However, the above referenced conventional method for forming the cobalt silicide layer is somewhat complicated due to the two heat treatment processes although the cobalt silicide layer having relatively low resistance may be obtained.
A method of forming a cobalt silicide layer as an ohmic layer is discussed in Korean Patent Laid Open Publication No. 2004-17655, U.S. Pat. No. 5,998,873 issued to Blair et al., and U.S. Pat. No. 6,734,098 issued to Tseng et al. According to the above-mentioned Korean Patent Laid Open Publication, a simple method of forming a metal wiring with a cobalt silicide film can be performed without several heat treatment processes.
FIGS. 1A to 1C are cross sectional views illustrating the method of forming the metal wiring including the cobalt silicide film. Referring to FIG. 1A, after an insulation layer 3 including a contact hole 2 is formed on a substrate 1, a cobalt film 4 is formed on the insulation layer 3 and an inside of the contact hole 2 including on the sidewall of the contact hole 2.
Referring to FIG. 1B, a titanium film 5 is formed on the cobalt film 4 at a temperature of about 400 to about 750° C. by a chemical vapor deposition process. The titanium film 5 and the cobalt film 4 serve together as an ohmic layer. When the titanium film 5 is formed at the high temperature, a cobalt silicide film 6 is formed on a bottom face of the contact hole 2. After a titanium nitride film 7 is formed on the titanium film 5, a conductive layer 8 is formed on the titanium nitride film 7 to fill the contact hole 2. The conductive layer 8 is formed using tungsten, aluminum, etc.
Referring to FIG. 1C, the conductive layer 8 is partially etched to form a metal wiring 9 on the substrate 1. Here, the metal wiring 9 includes a plug that makes contact with the substrate 1 through the cobalt silicide film 6. As is shown in FIG. 1C, a portion of the cobalt film 4 remains under the metal wiring on the sidewall of the contact hole 2 and on the substrate outside the contact hole 2.
However, in the above-mentioned method of forming the metal wiring, the cobalt film 4 on the insulation layer 3 is not easily etched by a dry etching process so that the cobalt film 4 may remain beneath the metal wiring 9 on the sidewall of the contact hole 2 and on the substrate outside the contact hole 2 as discussed above. Alternatively, if the cobalt film 4 is wet etched, the metal wiring including 9 the plug may be damaged. In either case, leakage current may be generated (by the damaged metal wiring 9 or by the portion of the cobalt film 4 that remains beneath the metal wiring 9) despite the etching.
FIG. 2 is a graph illustrating leakage current distributions of metal wirings formed in accordance with a conventional method. In FIG. 2, I indicates a leakage current distribution of a metal wiring that includes a titanium film having a thickness of about 85 Å and a titanium nitride film having a thickness of about 250 Å, and II indicates a leakage current distribution of a metal wiring that further includes a cobalt silicide film having a thickness of about 50 Å besides the titanium film and the titanium nitride film. Here, the titanium, the titanium nitride and the cobalt silicide films are formed by a chemical vapor deposition process at a high temperature of about 400 to about 750° C. As shown in FIG. 2, the leakage current of II is larger than the leakage current of I because a portion of a cobalt film, which is formed beneath the metal wiring to form the cobalt silicide film, remains beneath the metal wiring although a wet etching process or a dry etching process is executed to remove the remaining portion of the cobalt film beneath the metal wiring. Therefore, electric characteristics of a semiconductor device including the cobalt silicide film may be deteriorated.